Ocean thermal gradient: How energy resides
It is the temperature gap, not the heat, that counts
What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Sensible heat stored in the sunlit surface layer of tropical oceans, usable only because water about a kilometre down stays near 4 °C. The useful quantity is the temperature gap, typically around 20 °C, not the heat itself.
Form Sensible heat (a temperature difference between warm surface water and cold deep water) Origin Indirect solarHow it got there
Fusion in the Sun Sunlight Absorption in the top tens of metres of tropical seas Warm surface layer at 25-30 °C Cold polar water sinking and spreading at depth (about 4 °C) Vertical temperature gap of about 20 °CHow concentrated it is
| Measure | Value | Note |
|---|---|---|
| Temperature difference needed | about 20 or more °C (surface vs 800-1,000 m depth)1 | Surface around 25 °C, deep water about 4 °C at 1 km. |
| Maximum (Carnot) efficiency | 7% (theoretical ceiling)1 | Pumps then consume about 20-30% of gross output. |
| Power density of the resource | 0.26-0.32 W per m2 of sea surface4 | Modelled for Pacific island waters. |
| Seawater flow for a 100 MW plant (NASEM) | 200-300 m3/s3 | IRENA gives 750 t/s for the same size, but its own figure for a 10 MW plant (100,000 m3/hour) scales to about 280 m3/s, in line with NASEM. Either way the flow rivals a large river. |
| Seawater flow for a 100 MW plant (IRENA) | 750 tonnes per second1 |
Tropical seas absorb sunlight in a thin warm layer that floats on top of cold, dense water that sank near the poles and spread along the ocean floor. An OTEC plant uses the warm water to boil a working fluid with a low boiling point (usually ammonia), drives a turbine with the vapour, and condenses it again with cold water pumped up from 800-1,000 m.
Because the temperature gap is only about 20 °C, thermodynamics caps conversion at about 7%, and pumping eats a fifth to a third of what is generated. The energy is therefore extremely dilute: enormous volumes of water must pass through heat exchangers for each megawatt, which is why cold-water pipes for a 100 MW plant would be 10 m or more in diameter and about 1,000 m long (IRENA).
Sources
- Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
- An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments, chapter on ocean thermal energy National Academies Press, 2013
- Spatial and Temporal Variability of Ocean Thermal Energy Resource of the Pacific Islands (Posterari et al., Energies 17) MDPI (via PNNL Tethys), 2024